Look at the Lewis electron dot structures. You may need to draw them out to see this. I may be able to find some sites that would show the Lewis structure if you can't draw them out. At any rate, the difference in bond angles is due to the unpaired electrons (the lone pairs). Lone pair electrons take up more space than shared electrons and they "push" on the opposite site so those elements opposite to the lone pairs are closer together; hence the angel is less.
NH4^+ has no long pairs so it is the closest to the tetrahedral angle of 109o